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Abstract

We demonstrate an alternative approach for attosecond polarization gating. A setup composed of four quartz wedges and a quarter-wave plate allows an easy adjustment of the temporal gate-width and of the total dispersion. A numerical simulation of the pulse propagation beyond the carrier-envelope approximation enables a calibration of the setup and provides a flexible choice of the desired temporal polarization. An electron imaging spectrometer is used to measure the electron momentum distribution resulting from the ionization of xenon with our optical gated laser pulses. This allows us to measure the orientation of the polarization plane in the most intense temporal slice of the laser pulse. We compare the experimental results to theory and we numerically show the robustness of the method against non-ideal laser parameters.

Figures (11)

Schematic representation of the whole experimental setup. The polarization gating setup including four quartz wedges and an achromatic quarter-wave plate is indicated. The third wedge (green) is translated to vary the gate width, while the second and the fourth (red) fix the dispersion. (a) The polarizer and the spectrometer are used to perform the calibration of the wedge insertion. (b) The achromatic half-wave plate is used to measure the orientation θ of the linear polarization gate with respect to the lab coordinates.

Schematic illustration of our polarization gating setup. The optical axes of the different quartz wedges (labelled ne) at ± 45° are represented by blue lines. The fast axis of the achromatic quarter-wave plate (red) makes an angle ψ with the horizontal plane. Schematic representation of the laser pulse electric field polarization is given before (top left) and after (bottom right) the setup for L = 168 μm of quartz and for ψ = 0°. The “linear” part of the pulse, which shows an ellipticity below 0.15, is highlighted in green.

Polarization ellipse and its orientation with respect to the lab frame of reference. Definitions: X and Y are the lab coordinates and lie in the horizontal and in the vertical planes, respectively; a and b are the length of the major and minor axes, respectively; x and y are the principal axes of the ellipse, rotated by an angle θ from the lab coordinates; A and B are the projections of the ellipse on the lab coordinates; and α and β are angles that are intermediate steps in the calculation (see Eqs. (8)-(11)).

(a) Definition of the delay (τd) between the o- and e-waves in quartz. (b) Definition of the gate width (τgate) and the threshold ellipticity (εth). (c) Delay and gate width computed for the simulation parameters of the next section (see Fig. 5).

Orientation θ of the most intense part of the polarization gated pulse as a function of the quartz thickness. The angle θ is unwrapped for a better visualization. Dots: measurement for three different experimental days. Solid line: simulation with the ideal parameters of Fig. 5. Dashed line: simulation with the experimental spectrum (Fig. 10), no dispersion compensation and the real wavelength-dependent behavior of the quarter-wave plate.

Robustness of the polarization gated pulses against experimental flaws and sources of error. The left column shows contour plots of the ellipticity ε(t). The central column shows intensity profiles in arbitrary units and the right column contains the orientation of the major axis θ(t) in the lab coordinates. (a) Ideal conditions (repetition of Fig. 5) (b) The input polarization is linear at 5° from X. (c) The fast axis of the quarter-wave plate is at ψ = 5°. (d) Experimental spectrum. (e) Dispersion is not compensated. (f) Real wavelength-dependence of the quarter-wave plate. (g) Spectral phase is distorted by higher-order terms: −80 fs3, −40 fs4 and + 200 fs5.

Experimental ((a) and (b)) and simulated ((c) and (d)) projections of the laser spectrum on the lab coordinates. By a simple fit to theory, the zero insertion is found on the experimental delay stage. Notice the symmetry of the traces around L = 0.

Two-dimensional photoelectron spectra induced by ultrashort polarization gated laser pulses for three different orientations of the gate. The electric field in the gate (ε<0.15) is represented in green. The laser propagates in the + Z-direction and the dc-field is directed along + X. (Left) The gate is along the Y-axis. (Center) The gate is oriented at 45°. (Right) The gate is polarized along the X-axis.